Over the past few years there has been an increased interest in the ability to place defibrillation leads in the cardiac veins, and in particular in placing small diameter defibrillation electrodes in more apical locations within the cardiac venous system. For example, U.S. Pat. Nos. 5,755,766, 5,803,928, and 5,755,765 all disclose cardiac vein leads adapted to be placed within the cardiac venous system by means of a guidewire passing through a portion of the lead or passing through a guide mounted to the lead. In the context of these leads, the guidewire is first advanced through the cardiac venous system to the desired point of implant and the lead is thereafter advanced down the guidewire, to its desired ultimate location. In addition, it has been proposed to provide cardiac vein leads with a preformed bent tip, as a substitute for a guidewire, as disclosed in U.S. Pat. No. 5,531,781. It has additionally been proposed to provide channels to allow blood flow through the coronary veins in the context of a cardiac vein lead, as disclosed in U.S. Pat. No. 5,800,495. U.S. Pat. Nos. 5,755,766, 5,803,928, 5,755,765, 5,800,495 and 5,531,781, cited above, are incorporated herein by reference in their entireties.
In addition to the placement of electrodes in the great cardiac vein as generally as described in the above-cited patents, it has also been specifically proposed to place cardiac defibrillation electrodes in other cardiac veins. As described in Gray""s Anatomy, Bounty Books, 1977, the anterior cardiac veins collect blood from the anterior surface of the right ventricle and include the vein of Galen, running along the right border of the heart. The posterior cardiac vein (also referred to as the middle cardiac vein) is described as extending from the apex of the heart to the base of the heart, lying along posterior intraventricular groove (i.e. adjacent the septum). The left cardiac veins are described as collecting blood from the posterior surface of the left ventricle and opening into the lower border of the coronary sinus. A system for deploying electrodes in several of these locations is described in U.S. Pat. No. 5,797,967, also incorporated herein by reference in its entirety, wherein placement of the lead is accomplished by passing the lead through the great cardiac vein, to the apex of the heart, and thereafter passing it in an ascending fashion upward through the anterior or posterior cardiac vein. Placement of an elongated electrode in the middle cardiac vein for purposes of anodal hyperpolarization to enhance cardiac function, following the same implantation procedure, is disclosed in U.S. Pat. No. 5,800,464 issued to Kievel, also incorporated herein by reference in its entirety. U.S Pat. No. 5,935,160 discloses placement of a lead in the xe2x80x9canteriorxe2x80x9d or xe2x80x9cposteriorxe2x80x9d cardiac veins, however, the illustrated positions of the electrodes appear to correspond to the great cardiac vein and to a left cardiac vein, as described in Gray""s Anatomy. 
The present invention is directed toward a cardiac defibrillation system including a first, apical defibrillation electrode located in the cardiac venous system, passing around the apex of the ventricle and a second cardiac defibrillation electrode located in the superior vena cava. The system may also include additional defibrillation electrodes, for example a subcutaneous electrode in the form of the conductive housing of an associated implantable cardioverter/defibrillator. The apical electrode may be placed by passing the lead first into the coronary sinus and thereafter either through the great cardiac vein toward the apex of the heart and thereafter upward toward the base of the heart through the middle cardiac vein (lying along the intraventricular groove, adjacent the septum) or through an anterior cardiac vein. Alternatively the electrode may be placed by inserting the lead into the middle cardiac vein and passing it toward the apex of the heart and thereafter passing it upward through the great cardiac vein or an anterior cardiac vein toward the base of the heart. The superior vena cava electrode may be located on a separate lead and may be a conventional superior vena cava type electrode or may be located on the same lead as the apical electrode. Defibrillation pulses are preferably delivered between the apical electrode and the superior vena cava electrode. A third electrode, typically in the form of a subcutaneous electrode which may be the conductive housing of the associated implantable cardioverter/defibrillator may be employed in conjunction with the SVC and atrial electrodes in either a simultaneous pulse fashion wherein the subcutaneous housing is coupled in common with either the apical or SVC electrode or in a sequential pulse fashion wherein two pulses are delivered in sequence, one pulse delivered between the SVC and atrial electrode, a second pulse delivered either between the apical electrode and the subcutaneous electrode or between the SVC electrode and the subcutaneous electrode.
In particular, the inventors have determined that a defibrillation pulse delivery vector between an SVC electrode and an apical electrode located at least partially in the middle cardiac vein provides an opportunity for an improvement in defibrillation energy thresholds as compared to a corresponding pulse delivery vector between a right ventricular defibrillation electrode and an SVC electrode.